Researchers have only recently found the longest large mammal migration in the continental United States: Mule deer migrate 150 miles (241 kilometers) in western Wyoming each year. And it's no easy task for them—barriers include highways, fences, tough terrain, and bodies of water. In this video by Joe Riis, a National Geographic grantee and regular contributor, see the modern-day obstacles mule deer overcome to make the migratory trek that they likely have been making for generations.